I'm new too - 3 years and counting.
I've attached graphic about the things that make up a grow environment. You've covered a few of them but should work through the list to get things working better.
Light - is low. For a plant of that age, cannabis will thrive at double the amount of light that you're giving it. Since you're not using a meter, I have little faith in the readings but it's "run whatcha brung" so to speak so with go with it. The light saturation point for cannabis is considered 800-1000µmol. You grow is well below that and chracteristics of plants that aren't getting much light are that they are tall, with not a lot of branches or laves and have significant space between the nodes. I glanced briefly at your photos and they're not as tall and slender as some, they will benefit when you get your PPFD up. A grow at 1kµmol will yield, roughly, 80% more than a grow at 500 (the relationship is not quite linear) but you're a little behind the power curve so maybe you'll only get 50 or 60% more by turning up the dial. My recommendation would be to go to say, 650 µmol, check your plants and, if they're doing OK (they should be) go to 700 the day after.
A meter will tell you how much light your plants are getting. The plant will tell you how much light is can use and, if you give it too much light, the leaves will "canoe" or "taco" or they'll rotate from horizontal to vertical. They'll start to move in a few minutes so, up the light in the AM, hang out for a while, then go back in the tent every 30 mins for an hour or two. Take pictures at the start and then compare. If there's too much light, they'' let you know.
You may see them start to rise from horizontal toward the vertical - not turning vertical but actually rising up. That's referred to as "praying" and it's considered a good sign.
Leaves dropping - I'm a hydro grower so I don't know soil but, along the way, I've picked up that leaves that have tips that point down can be caused by overwatering or by too much nitrogen. Hopefully a soil grower can chime in but that's where I'd start to look.
Yellow coloration - it could be not enough nutrients but it could also just be that those leaves re dying off ("senescence"). The dark yellow is a sign of a leaf that's ready to drop off by the light green/yellow could be from not enough nutrients. If so, congratulations because "no one" gives their plants too few nutes. Growers tend to slather on the the nutrients thinking that plants will grow better but they don't. That's when you get toxicity.
How about providing info about the rest of the grow environment? Someone here should be able to provide info about nute levels and soil. As a hydro guy, I'm pretty useless re. soil.
[checks out the plant pix again]
Nice manifold. I been meaning to do that but I chickened out!
Re. light levels and leaves - your plant doesn't have "a small number of large leaves" (not enough light) but they're not "compact and bushy" which is what you see when plant has had "lotsa light". More light now=more weed later and also easier to harvest, too, because the branches are closer together.
OK, VPD - VPD is for plants what "feels like" is for humans.
Extremes of temp and humidity are hard for us to survive in because it's hard to keep our body temps in range once we get above, say, 80° and 60% or below 65° and 50% (or so).
Plants have similar limitations.
VPD is one number that represents a range of temp + RH values. Just like "feels like" does.
At 1.3, your VPD is a little high so if you can get a little more moisture in the air, that will get you down to 1.2. Not a big issue but 1.3 is a tad high at this stage.
You cannot set VPD properly without knowing leaf temperature. The AC Infinity software has a place to enter the "leaf temperature offset" which is typically -2°. If you haven't entered a value there - there could be a default value, howewver - your VPD is wrong and that could be a reason why you've got some yellowed/yellowing leaves. "could be".
Don't mean to sound harsh, I hope you don't take it that way. That's a nice looking plant and the manifold looks great. It's great you're spending the time and $$ to measure light, VPD, and RH and temperature. Those a very important factors to nail down and, with a couple of tweaks, you'll have a really good harvest.
First steps - get that light turned up (that's how plants make food) and then get the nutrient issue squared away.
